As the main propulsion or in addition to the main propulsion devices, vessels and corresponding floating structures having to stay in place for working utilise propulsion devices rotatable and controllable in a known manner for this staying in place. Such vessels are e.g. drilling ships used for oil and gas prospecting, various research vessels and vessels servicing underwater structures. Also oil and gas drilling rigs, service ferries and corresponding are known having been provided with such rotatable propulsion devices. Such vessels and corresponding floating structures are intended to stay in place for working for very long periods of time, whereby, inter alia, the servicing and repair possibilities of the propulsion devices constitute a considerable problem, because it is not often possible to take these vessels to the dockyard for servicing and repairs but they have to be performed on the spot.
Particularly for servicing and repairs, e.g. U.S. Pat. Nos. 6,375,524 and 6,439,936 earlier described arrangements in which, in the structures of a vessel, a shaft or a pit extending through the whole vessel in the vertical direction, open from its both ends, i.e. from top and bottom, is constructed for a propulsion device. For the propulsion device, a watertight propulsion machine room (“Canister”) is constructed which usually includes a drive motor and a gearbox, a control unit, cooling, hydraulics etc. of the propulsion device. The propulsion machine room is installed in the above-mentioned shaft in which it is movable in the vertical direction. When the propulsion machine room is in the shaft in the lower position, i.e. when the propulsion device is in its operating position located below the vessel bottom, sealing surfaces between the shaft and the propulsion machine room are closed, whereby water cannot rise in the shaft. When the propulsion machine room and the propulsion device are lifted into the service position, the sealing surfaces open and water is able to rise in the shaft. Due to this, the propulsion machine room with its propulsion device has to be lifted totally above the vessel waterline in order to be able to perform required servicing and repair measures. Then, the shaft is filled with water up to the waterline, whereby the use of the service space is limited as water heaves in the shaft. Water is not removed from the shaft until the propulsion machine room with its propulsion device is lowered down so that the sealing surfaces close.
A disadvantage of this kind of prior-art arrangement is, inter alia, that when the structure is lifted in the upper position, the unit is awkward to support. Wind and rough seas create limitations for the retraction of the Canister. In rough wind and seas, it is not thus possible to service the propulsion devices, which might impede or even prevent the operation of the vessel. A vessel, which is provided with arrangements of the Canister type, cannot be run when the propulsion machine room is in the upper position. When moving the propulsion device (Canister), the lifted mass is extremely large, because in the system the whole machine room is lifted with its propulsion device and required components. The lifting distance is also great, because the whole structure alongside the propulsion device has to be lifted above the waterline. Such a propulsion machine room is extremely difficult to realise in vessels low of their structure. The propulsion machine room being lifted within the hull, the open shaft constitutes a discontinuity point with the bottom of the vessel or floating structure causing additional resistance when in motion.